Conventionally, in order to impart various kinds of functions to a surface of a molded article containing an organic polymer compound, an etching treatment, a UV ray treatment, a chemical vapor deposition treatment, a plasma treatment and the like have been carried out. For example, a molded article formed by using an organic polymer compound such as a fluororesin or a polyolefin resin has low wettability of the surface and is difficult to be bonded by using an adhesive. Therefore, an etching treatment or a plasma treatment has been performed as a treatment to improve the adhesive property of the surface of the molded article.
A fluororesin, for example, is excellent in chemical resistance, weather resistance, heat resistance, electrical insulation properties, and surface characteristics, and is currently used as an industrial material. However, there is a problem that a fluororesin has high chemical stability and is difficult to be bonded to different kinds of materials because it has extremely strong bonds between carbon atoms and fluorine atoms. Meanwhile, a polyolefin resin is highly versatile, but has a problem of difficulty of its adhesion to different kinds of materials since it has low surface polarity. Therefore, methods for modifying the surface of a molded article containing an organic polymer compound have been proposed.
As a method for modifying the surface of a molded article containing, for example, a fluororesin among organic polymer compounds, an etching treatment of applying a solvent or the like containing a sodium-naphthalene complex to the surface of a fluororesin material has widely been carried out so far. However, since an etching treatment method causes discoloration of a part to which the solvent adheres to dark brown color, it is undesirable in terms of the appearance of the product in the case where the discoloration is visible in the appearance. Further, metal sodium may possibly remain on the surface of the fluororesin material, and it is not preferable depending on applications. For example, application of the molded article to a member which is to be brought into contact with a pharmaceutical product in a container for enclosing a pharmaceutical product is undesirable.
To solve such problems of an etching treatment, methods for modifying the surface of a molded article containing a fluororesin by carrying out a plasma treatment on the surface are proposed (for example, refer to Patent Documents 1 and 2).
Patent Document 1 discloses a surface modification method including physical modification of implanting ions of plasma in the surface of a fluororesin-based molded article to roughen the surface of the molded article by applying negative voltage to the molded article, and chemical modification of replacing fluorine atoms in the surface of the molded article with atoms other than fluorine atoms. In this method, as for the case of carrying out atmospheric-pressure plasma irradiation, output power of a plasma irradiation source is set to be 10 to 1000 W and plasma irradiation time is set to be 5 seconds to 60 minutes. In the section of “Examples,” it is described that the adhesion strength is 0.5 to 9.1 N/mm in the case where an epoxy-based adhesive is applied to the surface of a polytetrafluoroethylene (hereinafter, referred to as PTFE) sheet to which plasma irradiation is carried out at 300 W for 10 minutes under a reduced pressure of 0.5 or 1 Pa and then SUS304 is stuck thereto.
Patent Document 2 describes a surface-modified fluororesin film having a uniform thin film layer formed by graft-polymerizing a monomer containing a reactive unsaturated group such as acrylic acid on at least one surface of the fluororesin film layer, in a state where electric charges to be applied to the fluororesin film layer are eliminated while plasma irradiation is being carried out under predetermined conditions. Further, Patent Document 2 describes an article obtained by self-bonding an adherend which is directly bonded to this surface-modified fluororesin film by curing a thermosetting resin or rubber on the surface-modified fluororesin film.
Patent Document 3 discloses a fluororesin molded article having a content of fluorine atoms in the surface layer and a center line average roughness Ra that are adjusted by an ion implantation treatment, and having a predetermined number of pinholder-like fine projections per unit area.
Incidentally, it is known that PTFE among fluororesins turns to a gel at the melting point, and cannot generally be molded by injection molding or extrusion molding. Therefore, a sheet-like molded article of PTFE (PTFE sheet) can generally be obtained by compression molding a columnar molded material and cutting the surface of the material. It is known that when a peeling test is carried out on a composite which is obtained by subjecting the surface of the PTFE sheet obtained in such a manner to a plasma treatment and bonding the PTFE sheet to an adherend, the PTFE sheet is sometimes easily separated while the thin layer of the surface of the PTFE sheet remains bonded to the adherend. This is probably attributed to the following: the surface layer separation of the PTFE itself is caused due to the low strength of the surface part of the PTFE sheet resulting from the effect of the cutting treatment at the time of molding, although the bonding effect is obtained by the plasma treatment on the surface of the PTFE sheet.